1. Field of the Invention
The present invention relates to a semiconductor device, especially a bipolar transistor, and a method of manufacturing the same.
2. Description of the Prior Art
Recently, bipolar transistors and integrated circuits with bipolar transistors mounted thereon have been utilized in various application fields. The bipolar transistor is advantageous over the field effect transistor in terms of high speed performance and high breakdown voltage performance, and its application is expanded for communication devices, storage systems, etc. Conventional examples of a bipolar transistor capable of attaining high speed and high breakdown voltage include a silicon-germanium heterobipolar transistor using the selective epitaxial technique (Patent Literature 1). FIG. 3A and FIG. 3B show the distribution of impurities (FIG. 3B) and that of germanium (FIG. 3A) in a principal portion of a conventional transistor. In both FIGS. 3A and 3B, the axes of abscissa are illustrated in a correlative manner to make it easy to understand how Ge distribution and impurity concentration are correlated with each other. In the drawings, the impurity concentration of a low concentration collector layer is adjusted depending on the purpose of use of a transistor. More particularly, in the case of a transistor aiming at a high speed, a high concentration is attained by, for example, ion implantation, while in the case of a transistor for which a high voltage resisting characteristic is important, a low concentration is ensured. The distribution of germanium is designed so as to cover the base region, and a heterointerface comprising a silicon/silicon-germanium junction is formed in the vicinity of an emitter-base junction. In a heterobipolar transistor, a change of a forbidden band at a hetero interface in the vicinity of an emitter-base junction restricts a hole current flowing from base to emitter and brings about the effect of improving the current gain. On the other hand, on the collector side, it is likely that the aforesaid change of the band gap will obstruct the transistor operation, and therefore the heterointerface is designed so as to be spaced a certain distance from a collector-base junction. If the heterointerface is present near the collector-base junction, there is a great possibility that the heterointerface may be positioned within a p-type base layer under the influence of base impurity diffusion caused by heat treatment or the like during the formation of a transistor. In this case, all discontinuous quantity of the band gap appears as barrier in a conduction band, greatly impeding the conduction of electrons, with a consequent great decrease of the current amplification factor and deterioration in high-speed operation of the transistor. In the case of an npn-type bipolar transistor using silicon-germanium, if a heterointerface is present sufficiently on the collector side rather than collector-base junction, all discontinuous quantity of the band gap appears on a valence band side and therefore the above-mentioned problem does not arise in at least a low current operation. However, also in the conventional heterobipolar transistor thus designed, there still exists the problem that the influence of a heterointerface on the collector side greatly impedes the transistor operation in the case where a high current operation is needed. FIG. 4 is an energy band diagram in low and high current operations of a conventional npn-type heterobipolar transistor. A concrete construction of npn heterojunction is shown in the upper portion of the figure. Usually, when a high current is applied to an npn-type transistor, many electrons are accumulated in collector-base junction which is attributable to a finite carrier speed, canceling a fixed charge in a depletion layer in the junction. This phenomenon causes an increase of the base width, i.e., Kirk effect. However, in the case of a bipolar transistor having a heterointerface also on the collector side, the heterointerface and the collector-base junction approach each other upon occurrence of Kirk effect and the foregoing conduction band barrier occurs in the heterointerface and greatly impedes the transistor operation. FIGS. 5 and 6 schematically show collector current dependence of current gain and cut-off frequency respectively of a heterobipolar transistor. In both figures, the axes of abscissa represent collector current and the axes of ordinate represent current gain (FIG. 5) and cut-off frequency (FIG. 6). In each figure, a thin line characteristic is of Si BJT and a thick line characteristic is of SiGe HBT. Further, the value of collector current usually employed is shown as “working current (current of use).” As shown in the figures, in the heterobipolar transistors, an abrupt lowering of current gain and that of cut-off frequency are observed and thus transistor characteristics are deteriorated in comparison with the ordinary type of bipolar transistors. The lower the collector impurity concentration, the more conspicuous this phenomenon. This phenomenon is apt to occur particularly in a transistor which aims at attaining a high breakdown voltage. As shown in the figures, satisfactory characteristics are not obtainable in the vicinity of the working current.
[Patent Literature 1]
Japanese Patent Laid-Open No. Hei 10-79394